1. Field of the Invention
The present invention relates to a braking force control device, and in particular, to a braking force control device which controls the braking force applied to a wheel on the basis of the road surface xcexcslope or a physical value equivalent thereto.
2. Description of the Related Art
Conventionally, antilock brake (ABS) control devices as follows have been proposed. Time series data of wheel speed is detected each time a predetermined sampling cycle occurs. On the basis of the detected time series data of the wheel speed, the history of changes of the wheel speed and the history of changes of the wheel acceleration are computed. On the basis of these computed values, a braking force slope (the slope of the braking force with respect to the slip speed) is estimated. The braking force applied to the wheel is controlled on the basis of the estimated braking force slope.
Braking force of a tire is generated by the slip between the tire and the road surface. In other words, the braking force of the tire is generated by the difference between the speed at which the tire advances (the advancing speed of the vehicle body) and the circumferential speed of the tire. Usually, an ABS control device computes the wheel slip and the wheel deceleration on the basis of a wheel speed signal, controls the increase, maintenance and decrease of the brake fluid pressure in accordance therewith, and prevents locking of the wheel.
As shown in FIG. 1, the characteristic of the frictional force between a tire and the road surface (the so-called xcexc-S characteristic) varies in the direction of arrow X and the direction of arrow Y along the xcexc-S characteristic at the time of pressure increase of ABS control. At the time of pressure reduction, the xcexc-S characteristic cycles so as to fall in the xcexc direction (the direction of arrow Z).
In order to effectively carry out ABS control by utilizing the xcexc-S characteristic of the tire, at the time of pressure god increase, when there is slippage which moves away from the xcexc peak, pressure is increased immediately (the direction of arrow X), and in a vicinity of the xcexc peak, the pressure increase amount is suppressed to a slight amount (the direction or arrow Y) or is maintained, such that the time of staying in the vicinity of the xcexc peak is as long as possible. On the other hand, at the time of pressure reduction, it is necessary to restore the slip immediately (the direction of arrow Z).
In a conventional ABS control device, the threshold value for carrying out pressure increase or pressure reduction is set so as to match the characteristic of a general tire. Accordingly, a problem arises in that this threshold value is not always the optimal value of a given tire for a given road surface.
In order to overcome this problem, for example, Japanese Patent Application Laid-Open (JP-A) No. 7-165053 discloses a technique in which a friction force characteristic between the tire and the road surface is estimated, such that the ABS control ability is improved. In this conventional art, by utilizing the fact that the wheel acceleration generates a difference between the braking torque and the depressing force reaction force (the braking force applied to the vehicle), a slip ratio is determined such that the difference between the wheel acceleration and the vehicle deceleration becomes a predetermined value, and a target slip ratio is determined in consideration of the offset.
However, a wheel speed signal includes noise, and further, the vehicle acceleration is estimated from the wheel speed which includes the wheel slip. Thus, it is difficult to accurately compute the vehicle acceleration or the wheel acceleration. As a result, a problem arises in that the tire frictional force characteristic with respect to tire road surface cannot be accurately known. Further, in conventional methods, it cannot be judged what type of conditions there are at the xcexc-S characteristic at the time of ABS control. As a result, it is extremely difficult to judge whether pressure should be increased immediately or whether pressure should be increased slowly.
Further, in Japanese Patent Application Laid-Open (JP-A) No. 2000-118375, the applicant of the present application has proposed an antilock brake control device which realizes a maximization of the braking force by carrying out follow-up control such that an estimated braking force slope becomes a specific target value in a vicinity of zero.
In this conventional art, the braking force can be made to become substantially a maximum by effecting follow-up control such that the estimated braking force slope becomes a target value near zero at the time of braking while the vehicle is advancing god straight forward. However, in a case in which a slip angle arises, i.e., in a case in which lateral slip arises at the wheel due to braking while turning, when follow-up control is effected such that the braking force slope becomes a target value in a vicinity of zero, a problem arises in that there is the possibility that the limit of the tire generation force characteristic will be exceeded.
This problem will be described with reference to FIGS. 2 and 3. As shown in FIG. 2, for example, when a slip ratio xcexax in the longitudinal direction (the vehicle longitudinal direction) and a slip ratio xcexay in the lateral direction are greater than or equal to about 0.15, the tire generation force exceeds 100%. No further tire generation force can be obtained, and a total slip region is entered. Note that point A in FIGS. 2 and 3 is a point at which the braking xcexc slope (the braking force slope) xcex1x becomes 3 when the lateral direction slip ratio xcexay is 0.13.
Further, FIG. 3A illustrates the relationship between the longitudinal direction slip ratio xcexax and the braking xcexc (longitudinal direction braking force) at the time when the lateral direction slip ratio xcexay is 0.13. FIG. 3B shows the relationship between the longitudinal direction slip ratio xcexax and lateral xcexc (lateral force). FIG. 3C shows the relationship between the longitudinal direction slip ratio xcexax and composite xcexc (composite force). Further, 3A also shows the braking xcexc slope, and shows point A where the braking xcexc slope a, xcex1x=3.
In accordance with FIGS. 2 and 3, as the longitudinal direction slip ratio xcexax increases, the braking xcexc increases and the lateral xcexc decreases. The composite xcexc which is the synthesis thereof is saturated before point A is reached.
Namely, when the target value of the braking force slope is fixed to a specific value at the time when a lateral slip exists, regardless of the fact that the braking xcexc slope is in a state before the peak, the composite xcexc may reach the peak. In such a region, the lateral force may be reduced unnecessarily and the total slip region may be entered, which is not preferable from the standpoint of control stability of the vehicle.
The present invention is proposed in order to overcome the above-described drawbacks, and an object of the present invention is to provide a braking force control device which correctly grasps the road surface state and can carry out optimal control which is appropriate for the road surface state. Further, an object of the present invention is to provide a braking force control device which can improve control stability at the time of limit braking during turning.
In order to overcome the above-described problems, a first aspect of the present invention is a braking force control device comprising: wheel speed detecting means for detecting a wheel speed of each wheel of a vehicle; road surface xcexc slope estimating means for, on the basis of the detected wheel speed, estimating, for each wheel a slope of a coefficient of friction xcexc between the wheel and a road surface as a road surface xcexc slope; and control means for, on the basis of the road surface xcexc slope estimated for each wheel by the road surface xcexc slope estimating means, adjusting a braking force for each wheel by controlling the braking force of each wheel.
In accordance with the first aspect of the present invention, the road surface xcexc slope estimating means estimates the road surface xcexc slope of each wheel on the basis of the wheel speed of each wheel. Note that the road surface xcexc slope estimating means may estimate a physical amount which is equivalent to the road surfaced xcexc slope, e.g., the braking force slope, drive force slope, braking torque slope, drive torque slope, or the like. The control means adjusts the braking force between the respective wheels by controlling the braking force of each wheel on the basis of the road surface xcexc slope. Accordingly, the braking force is controlled while the state between the respective wheels and the road surface is estimated. Thus, appropriate braking can always be carried out in accordance with the state of the wheels and the road surface which is always changing.
In a second aspect of the present invention, the first aspect further comprises: control parameter generating means for generating a control parameter for braking force control; and control parameter correcting means for correcting the control parameter generated by the control parameter generating means, wherein the control means controls the braking force of each wheel on the basis of the control parameter corrected by the control sly parameter correcting means.
In accordance with the second aspect, on the basis of the road surface xcexc slope, the control parameter correcting means judges what position of the xcexc-S characteristic the tire is at, and corrects the control parameter such that the grip force of the tire is on the xcexc peak of the most exhibited xcexc-S characteristic. Note that when the tire is at the xcexc peak, it suffices to not correct the control parameter. Further, the control means effects control of the braking force by using the corrected control parameter, In this way, the control parameter can be set appropriately, and as a result, the control responsiveness and the vehicle stability at the time of braking force control can be improved.
At a time of increasing brake fluid pressure, in a case in which the road surface xcexc slope estimated by the road surface xcexc slope estimating means is greater than a first predetermined value, the control parameter correcting means can correct the control parameter to make a pressure increase amount of the brake fluid pressure large. Further, at a time of increasing brake fluid pressure, in a case in which the road surface xcexc slope estimated by the road surface xcexc slope estimating means is smaller than a second predetermined value, the control parameter correcting means can correct the control parameter to make a pressure increase amount of the brake fluid pressure small.
The control parameter correcting means can correct the control parameter such that the brake fluid pressure is maintained in a case in which the road surface xcexc slope estimated by the road surface xcexc slope estimating means is smaller than a third predetermined value.
At a time of reducing brake fluid pressure, in a case in which the road surface xcexc slope at a start of pressure reduction estimated by the road surface xcexc slope estimating means is greater than a predetermined value, the control parameter correcting means can correct the control parameter to make a pressure reduction amount of the brake fluid pressure small or to make a pressure reduction time short. Further, at a time of reducing brake fluid pressure, in a case in which the road surface xcexc slope at a start of pressure reduction estimated by the road surface xcexc slope estimating means is smaller than a predetermined value, the control parameter correcting means can correct the control parameter to make a pressure reduction amount of the brake fluid pressure large or to make a pressure reduction time long.
On the basis of the road surface xcexc slope at a start of pressure reduction estimated by the road surface xcexc slope estimating means, the control parameter correcting means can correct a slip threshold value which expresses a start of pressure reduction of the brake fluid pressure. At this time, when the road surface xcexc slope at a start of pressure reduction estimated by the road surface xcexc slope estimating means is larger than a predetermined value, the control parameter correcting means can effect correction such that the slip threshold value which expresses a start of pressure reduction of the brake fluid pressure is made large, and when the road surface xcexc slope at a start of pressure reduction estimated by the road surface xcexc slope estimating means is smaller than a predetermined value, the control parameter correcting means can effect correction such that the slip threshold value which expresses a start of pressure reduction of the brake fluid pressure is made small. Moreover, the control parameter correcting means can correct the slip threshold value which expresses a start of pressure reduction of the brake fluid pressure, and on the basis of this correction amount, can correct a slip threshold value which expresses a start of pressure increase of the brake fluid pressure.
The control parameter correcting means can correct a slip threshold value which expresses a start of pressure increase of the brake fluid pressure, on the basis of the road surface xcexc slope at a start of pressure increase estimated by the road surface xcexc slope estimating means. At this time, when the road surface xcexc slope at a start of pressure increase estimated by the road surface xcexc slope estimating means is greater than a predetermined value, the control parameter correcting means can effect correction such that the slip threshold value which expresses a start of pressure increase of the brake fluid pressure is made large. Further, when the road surface xcexc slope at a start of pressure increase estimated by the road surface xcexc slope estimating means is less than a predetermined value, the control parameter correcting means can effect correction such that the slip threshold value which expresses a start of pressure increase of the brake fluid pressure is made small. Moreover, the control parameter correcting means can correct the slip threshold value which expresses a start of pressure increase of the brake fluid pressure, and on the basis of this correction amount, can correct a slip threshold value which expresses a start of pressure reduction of the brake fluid pressure.
On the basis of a road surface xcexc slope, which is before start of braking force control and which is estimated by the road surface xcexc slope estimating means, the control parameter correcting means can correct a control parameter generated at the control parameter generating means.
When the road surface xcexc slope, which is before start of braking force control and which is estimated by the road surface xcexc slope estimating means, is smaller than a predetermined value, the control parameter correcting means can make the pressure reduction amount of the brake fluid pressure large and/or make the pressure increase amount small, and when the road surface xcexc slope, which is before start of braking force control, is greater than a predetermined value, the control parameter correcting means can make the pressure reduction amount of the brake fluid pressure small and/or make the pressure increase amount large.
In a third aspect of the present invention, in the first aspect, the control means controls a braking torque amount of each wheel on the basis of the road surface xcexc slope estimated for each wheel by the road surface xcexc slope estimating means.
In accordance with the third aspect, the control means controls the braking torque amount by, for example, increasing or reducing the brake fluid pressure on the basis of the road surface xcexc slope. Namely, even if there is no G sensor or G switch, by utilizing the road surface xcexc slope, it can be judged whether the tire is tending toward locking. Thus, the system can be simplified.
The braking force control device of the third aspect can further comprise: wheel acceleration detecting means for detecting a wheel acceleration on the basis of the wheel speed detected by the wheel speed detecting means, wherein the control means controls a braking torque amount on the basis of a relationship between the road surface xcexc slope estimated by the road surface xcexc slope estimating means and the wheel acceleration detected by the wheel acceleration detecting means.
The control means can control at least one of an amount of increase in braking torque and an amount of decrease in braking torque, on the basis of one of a wheel slip speed and a wheel slip ratio. By using not only the road surface xcexc slope and the wheel acceleration, but also the wheel slip speed or wheel slip ratio, the braking torque can be controlled more reliably, and thus, stability during braking can be improved.
When the road surface xcexc slope estimated by the road surface xcexc slope estimating means is less than or equal to a predetermined value, the control means can effect control to decrease the braking torque. When the four wheels of a vehicle lock simultaneously, road surface xcexc slope is a predetermined value or less. Thus, due to the control means carrying out control to reduce the braking torque, the grip of the tire can be restored. Further, the tendency to lock of the four wheels can be detected even if no G sensor or G switch is provided, and thus, the system can be simplified.
In a fourth aspect of the present invention, the braking force control device of the first aspect further comprises: lateral slip information detecting means for detecting lateral slip information of the wheel, wherein the control means controls a braking force of each wheel on the basis of the road surface xcexc slope estimated for each wheel by the road surface xcexc slope estimating means and the lateral slip information detected by the lateral slip information detecting means.
In accordance with the fourth aspect, the lateral slip information detecting means detects lateral slip information of the wheel. This lateral slip information of the wheel may be, for example, the direction of generation force of the wheel or the slip angle. Further, the direction of tire generation force substantially corresponds to the steering angle of the steering wheel of the vehicle. Thus, the lateral slip information detecting means may detect the steering angle of the steering wheel of the vehicle. In this case, a steering angle sensor may be used as the lateral slip information detecting means. The control means controls the braking force of the wheel for each wheel on the basis of the estimated road surface xcexc slope and the lateral slip information. In this way, by controlling the braking forces of the respective wheels in consideration of the lateral slip information, even in a case in which there is lateral slip, the total slip region can be prevented from being reached, and the control stability at the time of braking while turning can be improved.
The control means may be formed by a target value computing means for computing a target value of a friction state on the basis of the lateral slip information, and a braking force control means for controlling the braking force of the wheel such that a frictional state detected by a frictional state estimating means follows the target value. At this time, as the lateral slip of the wheel increases, it is preferable for the target value computing means to make the target value larger, and as the lateral slip of the wheel decreases, it is preferable for the target value computing means to make the target value smaller.